fix(spacing): connect section spacing to design token system

Wire .hbb-section to var(--hb-spacing-section) instead of hardcoded 6rem,
enabling hugoblox.layout.spacing (compact/comfortable/spacious) to take effect.

- Default to "spacious" (6rem) to preserve existing visual layouts
- Per-section override via design.spacing.padding still works (inline > class)
- Remove dead design.spacing frontmatter field from all templates
- Set correct layout.spacing token in each template's params.yaml

Closes #3314

modules/blox/assets/css/framework/components.css

@@ -2,8 +2,8 @@
 @layer components {
   /* Section styles for Hugo Blox blocks */
   .hbb-section {
-    padding-top: 6rem;
-    padding-bottom: 6rem;
+    padding-top: var(--hb-spacing-section, 6rem);
+    padding-bottom: var(--hb-spacing-section, 6rem);
   }
 
   .section-subheading {